This project will take advantage of the unique opportunity presented by the NIMH-funded Harvard study "Twin Study of Biologic Markers for PTSD" to explore right frontal brain hyperactivation as a potential biologic and familial vulnerability marker for post-traumatic stress disorder (PTSD). In non-PTSD studies, greater right than left frontal activation has been found to be associated with negative affect, and increased risk for depression, and has been interpreted as reflecting a "withdrawal" response disposition. Subjects will include Vietnam veteran monozygotic (Mz) twins discordant for combat exposure. Right frontal hyperactivation as a biologic marker for PTSD will be evaluated by comparing resting right vs. left frontal electroencephalograms (EEGs) to combat-exposed veterans with PTSD vs. combat-exposed veterans without PTSD. Right frontal hyperactivation as a familial vulnerability marker for PTSD will be evaluated by comparing the same measures in the (high-risk) unexposed Mz co-twins of exposed twins with combat-related PTSD vs. the (low-risk) unexposed Mz co-twins of exposed twins without combat-related PTSD. subjects will be diagnosed by a psychologist using the Clinician-Administered PTSD scale and Structured Clinical Interview for DSM-IV. Measures of asymmetrical frontal activation, indexed by alpha power density, will be derived from resting EEGs already being measured as an integral part of the Harvard PTSD Twin study. Statistical significance will be tested by analyses of variance and t-tests between a priori selected subgroups. Results are expected to advance our understanding of the constitutional vs. acquired nature of dispositional abnormalities in PTSD. Results may also have implications for the screening of persons at high risk for the development of PTSD upon exposure to military combat or other severe stressor.